----- Original Message -----
Sent: Sunday, September 17, 2006 6:12
PM
Subject: [FlyRotary] Re: ducted fan
questions
ing a ducted fan(not a shrouded
propeller).
Now that's just way too cool Richard
:-)
Was the fan running directly off the engine, with no
redrive? Did you feel that the performance was better than it would have
been with a traditional redrive and prop? If I get serious
about trying this, I'll certainly take you up on your offer to share
info.
Cheers,
Rusty
There are plenty
of ducted fans, look at the bypass stage of every turbofan engine. The
difference between a ducted fan and a shrouded propeller is the blade
solidity. This is the ratio of the fan cross section to the total blade area.
A turbofan is around "1" and a shrouded propeller, I have seen, may be as
low as .1 or less.
My fan with the
KAWA engine was running reduced at 3400RPM, in order to allow for more
solidity with the benefit of lower noise. When I changed to direct drive with
adjusted solidity at the same HP, the fan efficiency went from 70% down to
less then 50%, and the noise was almost unbearable.
The fan I flew had
six blades, and the test fan on direct drive had three blades, which were
smaller too.
BTW the diameter
was 26".
The conclusion is, if you aim for a
solidity of 1, or close to 1, for a given hp, there is only one fan size and
RPM. Now, what compromises can be made to arrive at a practical design? Your
imagination, and expectations, are the only
limits.
My 90+hp design
has nine blades at 26", giving a solidity of .6 . This is not very good,
however, I do not want to make a new shroud and I also have a given max RPM,
which locks me in. Nevertheless, the data would be as good as any, knowing the
deficiencies.
So much for now.
Should we discuss
this on another E-Mail? I don't want to bore our rotor head
friends.
Richard
Sohn
N-2071U